This invention relates to electrical capacitors. More particularly, the present invention relates to a new and improved capacitor having a dielectric material which has been stoichiometrically formed or treated to optimize or improve the electrical or functional characteristics of the capacitor, such as its capacitance value variation or tolerance, its change-in-capacitance per change-in-voltage (dC/dV), or its leakage current. Optimizing these characteristics achieves more reliable and predictable functionality, as well as precise operating characteristics, thereby making the capacitor more suitable for both analog and digital circuit functions when incorporated within an integrated circuit (IC).
Capacitors are commonly employed in ICs for a variety of purposes, such as to condition signals, to store electrical charge, to block DC voltage levels, and to stabilize power supplies. In memory ICs, a capacitor is used to hold enough charge to represent a detectable logic state.
Polysilicon is typically used to construct the electrode plates of the capacitor in a substrate of the IC. The diffusion and doping characteristics of polysilicon result in variable capacitance characteristics, in which the capacitance value varies relative to the voltage level applied to the capacitor and the temperature experienced by the capacitor. Despite the variable characteristics of polysilicon capacitors, the capacitance variation is not of primary concern in digital memory ICs. Memory capacitors are required only to accept charge, to hold some or all of the charge for a finite time period and then discharge, all in a reliable manner. Furthermore, since polysilicon is also used to fabricate other components of the IC, such as transistors and conductors, the plates of the capacitors can be formed simultaneously with the other components of the IC.
In analog and mixed signal circuit applications, on the other hand, capacitors are frequently used as impedance elements whose response characteristic must be linear. If the impedance of the capacitor is not fixed and reliably ascertainable, the capacitance response of the capacitor relative to voltage will vary non-linearly, causing unacceptable variations in the performance of the analog or mixed signal circuit.
Application specific integrated circuits (ASICs) sometimes combine analog circuitry with digital circuitry on the same substrate. In such applications, the fabrication of capacitors has become somewhat problematic. Polysilicon is a semiconductor, which is not the best material to use as an electrode to form a capacitor. A space charge layer forms in the doped polysilicon and adversely affects the capacitance vs. voltage response (linearity) and the frequency response of the capacitor. When a metal material is used for the electrode, however, no space charge layer exists.
Many contemporary ICs employ multiple layers of interconnects, as an adjunct of their miniaturization. Interconnects are layers of separate electrical conductors which are formed overlying the substrate and which electrically connect various functional components of the IC. Because of space and volume considerations in ICs, attention has been focused upon the effective use of the space between the interconnect layers. Normally the space between the interconnect layers is occupied by an insulating material, known as an intermetal dielectric (IMD). One effective use for the space between the interconnect layers is to form capacitors in this space using the interconnect layers. The previously referenced U.S. patent applications focus on different techniques for combining capacitors with the conductors of the interconnect layers to achieve desirable effects within the IC.
Because the conductors of the interconnect layers are of metal construction, the capacitors formed between the interconnect layers are preferably of a metal-insulator-metal (MIM) construction. A MIM capacitor has metal plates, usually formed on the metal conductors of the interconnect layers. The fourth and fifth above identified patent applications describe techniques for forming the metal capacitor plates with the conductors of the interconnect layers. The additional benefit of MIM capacitors is that they possess a higher degree of linearity and an improved frequency response. Unlike polysilicon capacitors, MIM capacitors incorporated within the interconnect levels are unobtrusive to the underlying digital components or circuitry.
The use of a MIM capacitor within the interconnect levels can also reduce the size of the overall IC structure because the digital circuitry exists under the capacitor, instead of beside it. Additionally, MIM capacitors are readily fabricated as part of the interconnect layers without a significant increase in the number of process steps or in the manufacturing costs. Connecting the MIM capacitors in the interconnect layers to the appropriate components of the IC is relatively easily accomplished by post-like or plug-like xe2x80x9cvia interconnectsxe2x80x9d that extend between the interconnect layers as needed.
However, even the more linear MIM capacitors are susceptible to non-linear performance under the influence of different electrical and physical conditions, and even relatively small deviations from the expected and desired performance may be sufficient to diminish the effective use of such capacitors in precise linear or analog circuits or in digital circuits.
It is with respect to these and other background considerations that the present invention has evolved.
The improvements of the invention relate to the discovery that the density of the film which represents the bonding network along with the incorporation of bonded and free hydrogen in capacitor dielectric materials, such as silicon nitride, silicon oxynitride or silicon dioxide, can induce undesirable electrical and functional effects. Such undesirable effects include excessive variability or tolerance in the electrical characteristics of the capacitor, excessive change-in-capacitance per change-in-voltage (dC/dV, referred to as the linear response) characteristics, and excessive leakage current.
The present invention makes use of this discovery by controlling the stoichiometry of the dielectric, the dielectric deposition conditions, the network bonding (i.e. density) and the hydrogen incorporation. In the case where the dielectric film is comprised of either an oxide or oxynitride film the density and hydrogen content can be manipulated by post deposition thermal anneals.
By eliminating excess hydrogen within the capacitor dielectric material and controlling the stoichiometry of the dielectric film, improvements or optimizations are obtained in the electrical and functional characteristics of the capacitor dielectric material. Such characteristics include the capacitors capacitance density, its linear response (dC/dV), and its leakage current.
The improvements of the present invention also relate to the recognition that the density and the amount of bonded and free hydrogen in a silicon nitride or silicon oxynitride capacitor dielectric material may be indirectly controlled and optimized. Such control and optimization achieve the desirable electrical and functional effects by controlling the ratio of silicon and nitrogen used in forming the silicon nitride or silicon oxynitride capacitor dielectric material.
These and other improvements are achieved in methods of fabricating a capacitor dielectric material in a capacitor. One fabrication aspect of the invention includes forming the capacitor dielectric material to contain hydrogen, and adjusting the composition of the capacitor dielectric material by controlling the stoichiometry of the capacitor dielectric material in order to adjust the amount of the hydrogen contained in the capacitor dielectric material and to obtain predetermined electrical or functional characteristics of the capacitor. Preferably, the capacitor dielectric material is formed from substances including silicon, nitrogen and hydrogen. Also, a stoichiometric ratio of silicon to nitrogen is preferably controlled to limit the amount of hydrogen in the capacitor dielectric material. The ratio of silicon to nitrogen is preferably approximately 1.0 or less, or even more preferably approximately 0.75.
Another fabrication aspect of the invention relates to forming the capacitor dielectric material to include hydrogen bonds by depositing the substances by using plasma enhanced chemical vapor deposition, breaking some of the hydrogen bonds by ionic bombardment to allow the hydrogen from the broken bonds to escape from the capacitor dielectric material, and controlling the amount of the hydrogen in the formed capacitor dielectric material by controlling the power and amount of ionic bombardment.
Still another fabrication aspect of the invention involves breaking hydrogen bonds to the capacitor dielectric material by applying at least one thermal cycle of temperature elevation and temperature reduction of the capacitor dielectric material, and controlling the amount of the hydrogen in the formed capacitor dielectric material by controlling the number and extent of the applications of thermal cycles to the capacitor dielectric material.
Broader aspects of the present invention apply to capacitor dielectric materials containing substances which, like hydrogen, have a tendency to promote ionic conduction in the capacitor dielectric material. Aspects of the present invention also relate to capacitors having dielectric materials formed by the method aspects of the present invention.
A more complete appreciation of the present invention and its scope, and the manner in which it achieves the above noted improvements, can be obtained by reference to the following detailed description of presently preferred embodiments of the invention taken in connection with the accompanying drawings, which are briefly summarized below, and the appended claims.